1. Field of the Invention:
This invention relates to an apparatus for controlling a continuously variable transmission (hereinafter called CVT) which is used for a transmission system in a vehicle.
2. Description of the Prior Art:
A CVT is used for a power transmission system which is to control continuously speed ratio e (=output side rotational speed/input side rotational speed) to improve the specific fuel consumption of an engine. The CVT has a belt which transmits the engine power from an input side pulley to an output side pulley, and a press force is applied on the input side pulley and the output side pulley, with usually the latter being pressed against the belt and being controlled by line pressure. Thus, the line pressure has to be controlled to necessary minimum value for avoiding the slip of the belt on the pulleys to ensure the transmission of torque and prevent the drive loss of an oil pump. Conventional control systems calculate engine torque Te from an intake throttle position .theta. and engine speed Ne irrespective of an altitude on which a vehicle travels and control the line pressure Pl in relation to this engine torque Te. However, while the vehicle travels on a highland, air concentration of the atmosphere is reduced and thereby the engine torque is reduced. Thus, when the line pressure Pl is calculated from the engine torque Te obtained irrespective of the atmospheric pressure Pa, the line pressure Pl becomes too large so that troubles such as drive loss of the oil pump or life shortening of each element like belt in CVT are brought about.